1. Field
The following description relates, in general, to dual rotor wind turbines and, more particularly, to a dual rotor wind turbine, which has two rotor assemblies and in which inlet and upper guide vanes are placed at lower and higher locations in front of each rotor assembly, and a rotor shaft of the rotor assemblies is placed downstream of a vertical shaft, so that the rotor shaft is offset from the vertical shaft.
2. Description of the Related Art
Generally, wind turbine systems are classified into horizontal axis wind turbine systems and vertical axis wind turbine systems. Further, a hybrid wind turbine system, which is a combined wind turbine system, has been proposed. The efficiency of a vertical axis wind turbine is approximately half that of a horizontal axis wind turbine. However, the vertical axis wind turbine has a low rotor rpm, so that it produces low levels of noise and vibrations. Therefore, the vertical axis wind turbines are preferably used in buildings and public facilities, such as schools and hospitals. Further, even though the vertical axis wind turbines are produced using parts and blades having a low precision, they can be used for a lengthy period of time and can effectively produce electricity, so that they can be used in independent, small-sized wind turbine systems. However, in order to provide a vertical axis wind turbine system for distributed power generation essential to environment-friendly regional development, it is required to reduce the weight of the turbine and realize desired price competitiveness of the turbine by increasing the turbine efficiency, thereby realizing compactness of the turbine and using fewer blades and parts.
Generally, the vertical axis wind turbines for distributed power generation, which produce low levels of noise and can be preferably used in a housing complex, are classified into Savonius drag type turbines and Darrieus lift type turbines. The drag type turbines have been developed in a manner in which the number of blades is increased and a vane is devised in order to increase the turbine efficiency. The lift type turbines have been developed to cross-flow turbines.
However, a jet wheel type vertical axis wind turbine, which is devised by installing an inlet guide vane in the inlet of the Savonius turbine, thus overcoming the low efficiency of the Savonius turbine and using the advantages of the Savonius turbine capable of producing torque at low rpm, has been proposed. In other words, the jet wheel type vertical axis wind turbine maximizes the positive torque parts in the blades of the Savonius turbine and produces low pressure zone in the negative torque parts in the downstream area of the inlet guide vane, thus removing the negative torque.
Described in detail, in the jet wheel type vertical axis wind turbine, an internal flow between blades in the downstream area of the inlet guide vane is prevented and high speed dynamic pressure, received from the inlet guide vane, is converted into static pressure in spaces between the blades in the downstream area of the inlet guide vane, so that a relatively low pressure produced by a high speed flow on a suction surface of the blade can be used to produce torque. Thus, it is required to reduce the cord length of the inlet guide vane. Further, an appropriate radius of curvature is provided to a flow streamline and the blade exit angles are set to realize a maximum distribution of incident angles of attack of rotor blades from the upstream area to the downstream area at a predetermined blade tip speed ratio.
Generally, the Savonius turbine produces torque using a drag force in an upstream blade and makes the flow pass through a downstream blade. However, as shown in FIG. 1, in a conventional jet wheel type turbine 100, a high speed incident flow condition capable of realizing an easy energy conversion is provided both by an inlet guide vane 120 and by a side guide vane 130, and the upper and lower surfaces of the turbine 100 are open, so that an inlet fluid can flow to the hub surface of blades 200 and increase the positive torque and reduce the negative torque, thus improving the turbine performance.
Further, a turbine blade structure for vertical axis wind turbine systems, which can increase the performance of the vertical axis wind turbine system, has been proposed in recent years. In the turbine blade structure, a sweep angle distribution is adapted to the blades 200 of the turbine rotor and each blade 200 is twisted to form a twisted shape, so that a rotating force can be continuously transmitted to the turbine 100 and a streamline from a radial direction to an axial direction can be easily formed.
However, in the jet wheel type turbine, in order to increase the efficiency of the wind turbine system, it is required to make the inlet rotor upstream wind surface area equal to the rotor surface area. Thus, the combined guide vane system having the inlet guide vane 120 and the side guide vane 130 is steered by the tail wing such that the guide vane system can be oriented into the direction of the wind. Therefore, the output performance of the small-sized jet wheel type turbine is increased. That is, at a rated wind speed, the turbine is steered by the tail wing such that the inlet guide vane 120 is oriented into the direction of the wind. At a wind speed over the rated wind speed, the tail wing starts to fold and all the guide vanes are rotated at regular angles relative to the direction of the wind. Therefore, the rpm of the turbine rotor is reduced, thus reducing the output power of the turbine rotor and preventing overload of the generator.
However, the rotating force typically acting on the inlet guide vane is stronger than the opposite directional rotating force acting on the side guide vane, so that, in order to steer the guide vane system in the same direction as the direction of the wind, a stabilizer and a rudder are required, both having large surface areas. Further, when a rotating force acts on a hinge due to abnormal spring elasticity caused by a pressure difference between the positive pressure surface and the suction surface of the rudder at a wind speed which is too fast, a difference in the torque balance is generated in the guide vane system, so that the guide vane system rotates to cancel the torque imbalance and is stopped. Therefore, during the guide vane motion, the load caused by a high wind acts on the rudder. Due to the torque unbalance of the guide vane system, the tail wing surface area and the weight of the tail wing are both increased, so that the jet wheel type turbine has a first problem of the guide vane system possibly leaning toward the tail wing and a second problem of the turbine structure having to be excessively reinforced due to the wind load acting on the rudder at a wind speed which is too fast.